A typical manufacturing method of a urea grease includes: mixing a base oil with isocyanate to prepare a first solution kept at about 60 degrees C.; mixing a base oil with amine to prepare a second solution kept at about 60 degrees C.; adding the second solution to the first solution with stirring; heating the obtained mixture to about 160 degrees C.; and subsequently cooling the mixture to the room temperature. However, this method takes time for manufacturing (i.e., synthetic reaction) and is likely to generate micelle particles (so-called lumps) formed of a thickener. Large lumps are known for deteriorating an acoustic property when the grease is used in a slide device such as a bearing. Further, since the thickener having a non-uniform structure formed of large lumps less contributes to an inherent performance of the grease, an efficiency of the thickener is decreased. In other words, a lot of thickener is required for obtaining a predetermined hardness.
Accordingly, a grease manufacturing method for inhibiting formation of large lumps and improving an acoustic property has been proposed (see Patent Literatures 1 and 2). The manufacturing method disclosed in Patent Literature 1 includes: a method of feeding an amine solution (or an isocyanate solution) with liquid drops having a diameter of 300 μm or less into an isocyanate solution (or an amine solution) using a spray nozzle; and a method of spraying the above solutions to each other for reaction. This manufacturing method restricts a particle diameter of each of lumps formed of the thickener (a urea compound) to less than 100μm (about several tens μm). The manufacturing method disclosed in Patent Literature 2 includes a method of applying pressure to an amine solution and an isocyanate solution using a pressure device to increase the pressure to a predetermined pressure, and mixing the solutions by colliding with each other for reaction. This manufacturing method restricts a size of each of lumps to a range from about several hundreds um to several tens p.m.